Substrates for magnetic discs, magnetic discs and process for producing magnetic discs

ABSTRACT

A magnetic disc substrate is provided, which includes a magnetic disc substrate body made of glass and is characterized in that a metal element capable of absorbing light is present in at least a surface portion of the magnetic disc substrate body, and a texture is formed on a surface of the magnetic disc substrate body. Ions of the metal element are dispersed in the surface portion of the magnetic disc substrate, or the metal element is contained in a composition of the glass constituting the magnetic disc substrate in the form of an oxide. The glass is preferably a crystallized glass a Li 2  O--Al 2  O 3  --SiO 2  based crystallized glass, which particularly preferably contains 65 to 85 wt % of SiO 2 , 8 to 15 wt % of Li 2  O, 2 to 8 wt % of Al 2  O 3 , 1 to 5 wt % of P 2  O 5  and 1 to 10 wt % of ZrO 2  and has lithium disilicate (Li 2  O·2SiO 2 ) as a main crystalline phase.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to substrates for magnetic discs,crystallized glass suitable for such magnetic discs, magnetic discs anda process for producing magnetic disc substrates.

(2) Related Art Statement

Principal constituent elements of a magnetic memory apparatus such as acomputer are a magnetic recording medium and a magnetic head formagnetically effecting recording and reproduction. As the magneticrecording medium, the flexible disc and the hard disc are known. As tothe hard disc, an aluminum alloy has been principally used as itssubstrate material. However, with the miniaturization of the hard discdrive, a floating amount of the magnetic head conspicuously decreases.Owing to this, extremely high accuracy has been demanded for theflatness and smoothness of the surface of the magnetic disc.

In general, it is necessary to suppress the maximum vertical height ofan uneven surface of the magnetic disc to a level not more than half thefloating amount of the magnetic head. For example, if the hard discdrive has an allowable floating amount of, for example, 75 nm, themaximum vertical of the uneven surface of the disc must be not more than38 nm. Particularly, it has been recently required that the center lineaverage surface height (Ra) of the uneven surface of a reading/writingzone of the magnetic disc substrate is not more than 20 Å. However, thealuminum alloy substrate has a small hardness. Therefore, even thoughthe substrate is polished with use of high precision grinding grains andworking machines, the polished surface of the substrate is plasticallydeformed. Consequently, it is difficult to produce a flat and smoothsurface at a high accuracy greater than a given degree. For example,even if the surface of the aluminum alloy substrate body is plated withnickel/phosphorus, the flat and smooth surface cannot be be obtained atthe above level.

Further, as the hard disc drive is miniaturized and made thinner, it hasbeen demanded that the thickness of the magnetic disc substrate is madesmaller. However, since the aluminum alloy has low strength and lowrigidity, it is difficult to maintain the given strength required fromthe specification of the hard disc drive while making, the disc thinner.Particularly, if the magnetic disc substrate is not more than 0.5 mm,the strength of the substrate becomes insufficient, so that thesubstrate may be warped or vibrated during high speed rotation andduring the start of rotation.

Magnetic resistance type heads (MR head) had recently begun to be used,and demand for reducing noise of the magnetic disc has been increasing.In order to reduce such noise, it is known to thermally treat a magneticfilm when or after the magnetic film is sputtered. In order toeffectively reduce the noise of the magnetic disc by the thermaltreatment, the thermal treatment needs to be effected at a temperaturenot less than 280° C. However, the thermal treating temperature cannotbe raised to more than 280° C. in the case of the aluminum alloysubstrate.

As the hard disc drive becomes more compact and thinner, there is astrong demand to decrease the thickness of the substrates for themagnetic discs. However, since the aluminum alloy has low strength andlow rigidity, it is difficult to make the discs thinner while the givenstrength required from the specification of the hard disc drive is beingmaintained. In particular, if the magnetic disc drive is worked to notmore than 0.5 mm, there is problems that the substrate is warped or thesurface of substrate is vibrated during a high speed rotation or at thetime of starting owing to insufficient strength of the substrate.

In order to solve the above problem, magnetic disc substrates made ofglass have been put into practical use in some cases. However, sinceparticularly high strength is required in the case of the substrate forthe HDD magnetic disc, chemically tempered glass or glass ceramics needto be used. When such a glass material is used, a magnetically recordingsurface having a very small Ra of not more than 20 Å can be formed.

Since glass actually has a low strength, it does not have reliabilitysufficient enough to be used as substrates for HDD type magnetic harddiscs. Chemically reinforced glass such as soda-lime glass is alsoknown. However, if such a chemically tempered glass is used as asubstrate for magnetic disc, alkaline metal ions contained in thesubstrate may be dissolved out to corrode a magnetic film.

On one hand, in order to prevent the head sticking phenomenon that themagnetic head slider is stuck to the surface of the magnetic disc whenthe magnetic disc drive is stopped, it is necessary to form such atexture on the surface of the magnetic disc that is constituted by bumpshaving heights of around 200 Å. On the other hand, as mentioned above,it is necessary to attain the flatness and smoothness of the surface toa high degree. For this reason, a reading/ writing zone and a landingzone are formed on the surface of the magnetic disc, and thereading/writing zone is finely polished to increase the magneticallyrecording density, whereas the texture is formed on the landing zone.

The above texture is formed by a photography process or an etchingprocess in the case of a soda lime glass (chemically tempered glass).However, these process are costy, and it takes a larger cost so as toparticularly decrease the area of tips of the protuberances constitutingthe texture.

Further, in the case of the aluminum substrate for the magnetic disc, ametal having a low melting point is sputtered upon the surface of thesubstrate, which is heated to form minute, semi-spherical protuberances.However, it is difficult and takes a large cost to form suchprotuberances only upon the landing zone.

Furthermore, as to a magnetic disc substrate made of crystallized glass,a texture is formed through polishing by utilizing difference inhardness between crystalline grains and the intergranular phaseconstituting the crystallized glass. However, according to this process,the texture was formed on both the landing zone and the reading/writingzone but it was impossible to form the texture on the landing zonealone.

SUMMARY OF THE INVENTION

Under the circumstances, the present inventors examined use ofcrystalline glass as a material for magnetic disc substrates. In thecrystallized glass, almost all alkaline metal ions contained exist inthe crystalline phase, whereas only a very small amount of them existsin the glass matrix. Therefore, such a problem does not occur that thealkaline metal ingredients dissolve out to corrode a magnetic film.Further, since the crystalline glass has small variations in terms ofhardness and bending strength as compared with the chemically temperedglass, the former has more excellent reliability, and is extremelypreferred particularly if the thickness of the magnetic disc substrateis as thin as 0.5 mm or less.

However, since the chemically tempered glass is entirely amorphous, thecenter line average surface roughness (Ra) can be reduced to around 6 Åafter polishing of the chemically tempered glass to use it as a magneticdisc substrate. However, since the hardness of the crystallized glassdiffers between the crystalline phase and the amorphous phase, fineunevenness is irrevocably formed between the crystalline phase and theamorphous phase even after the polishing. As a result, it was difficultto suppress the center line average surface roughness of the polishedsurface to not more than 20 Å.

For the above reasons, the present inventors discovered that if aspecific Li₂ O--Al₂ O₃ --SiO₂ based crystallized glass is used, thecenter line average surface roughness of its worked surface havingundergone the fine polishing could be reduced to not more than 20 Å.Consequently, the inventors specifically disclosed a magnetic discsubstrate using such a crystallized glass in the specification ofJapanese Patent Application No. 7-174,895.

However, even since then, a demand to record particularly a largervolume of information such as image information in a more compactmagnetic disc has become stronger with the development of themulti-media, so that increase in the recording density in the magneticdiscs has been demanded. As a result, it is requested that the centerline average surface roughness (Ra) of the reading/writing zone be notmore than 10 Å. With respect to a substrate made of general crystallizedglass and a magnetic disc substrate made of Li₂ O--Al₂ O₃ --SiO₂ basedcrystallized glass, it is difficult to satisfy the above requirement.With respect to non-crystallized glass and partially crystallized glasshaving a low crystallinity, it would be possible to control the centerline average surface roughness to not more than 10 Å after finepolishing. However, strength of these materials is relatively low sothat given strength suitable for the magnetic disc substrates cannot beobtained.

It is an object of the present invention to provide a process forforming a texture upon a desired portion (such as a landing zone ) of asurface of a magnetic disc substrate made of glass.

It is another object of the present invention to provide a process whichenables the formation of textures upon a number of magnetic discsubstrates in a short time with high productivity.

A further object of the present invention is to remarkably reduce thecenter line average surface roughness (Ra) of the surface of the Li₂O--Al₂ O₃ --SiO₂ based crystallized glass after the fine polishing whilemaintaining given crystallinity and strength so that flatness down toparticularly not more than 10 Å may be realized.

Further, it is a still further object of the present invention to obtaina polished surface having Ra of not more than 10 Å and to form a textureat an appropriate height on this surface with respect to the Li₂ O--Al₂O₃ --SiO₂ based crystallized glass, while maintaining givencrystallinity rate and strength.

A first aspect of the present invention is directed to a magnetic discsubstrate including a magnetic disc substrate body made of glass andcharacterized in that a metal element which absorbs light is containedin at least a surface area of the magnetic disc substrate body, and thata texture is formed at the surface of the magnetic disc substrate body.The first aspect of the invention also relates to a magnetic disccharacterized by including the above magnetic disc substrate and amagnetic film formed on the magnetic disc substrate.

The first aspect of the present invention also relates to a process forproducing the magnetic disc substrate, characterized by incorporation ofa metal element absorbing light into at least a surface portion of amagnetic disc substrate body made of glass and forming a texture byirradiating high energy light upon the surface of the magnetic discsubstrate body.

A second aspect of the present invention relates Li₂ O--Al₂ O₃ --SiO₂based crystallized glass which contains 65 to 85 wt % of SiO₂, 8 to 15wt % of Li₂ O, 2 to 8 wt % of Al₂ O₃, 1 to 5 wt % of P₂ O₅ and 1 to 10wt % of ZrO₂ and has lithium disilicate (Li₂ O·2SiO₂) as a maincrystalline phase.

The second aspect of the present invention also relates to a magneticdisc substrate made of the above crystallized glass and having a flatand smooth surface with a center line average surface roughness (Ra) ofnot more than 10 Å. The second aspect of the invention also relates to amagnetic disc including this magnetic disc substrate, an under filmformed on the flat and smooth surface of the magnetic disc substrate anda metallic magnetic film on the under film.

In order to produce the crystallized glass according to the secondaspect, a raw glass which contains 65 to 85 wt % of SiO₂, 8 to 15 wt %of Li₂ O, 2 to 8 wt % of Al₂ O₃, 1 to 5 wt % of P₂ O₅ and 1 to 10 wt %of ZrO₂ is crystallized by heating with the maximum temperature being680° C. to 770° C. during the production of the Li₂ O--Al₂ O₃ --SiO₂based crystallized glass.

The present inventors have been performing investigations to effectivelyform the textures upon the surfaces of the magnetic disc substrates madeof glass, particularly upon the surfaces of the magnetic disc substratesmade of crystallized glass. During these investigations, the inventorsfound out that when a metal capable of absorbing a light having a givenwavelength was contained in at least a surface portion of the magneticdisc substrate and a high energy light was irradiated upon the surfaceof the magnetic disc substrate, that surface portion of the substrateupon which the high energy light was irradiated was protruded. Further,the present inventors have found that if the dimension of the protrudedportions is appropriately controlled, such protruded portion would beextremely effectively utilized as a texture on the surface of themagnetic disc substrate or particularly as the texture on the landingzone of the substrate. The present invention has been accomplished basedon these findings.

At least the surface portion of the magnetic disc substrate could beappropriately protruded by incorporating the metal in at least thesurface portion of the magnetic disc substrate. Particularly, it is easyto make very small the top portions of the protuberances formed by theinventive process. When the total area of the top portions of theprotuberances is made very small like this, friction between themagnetic head and the surface of the magnetic disc substrate can belargely reduced.

The total area of the top portions of the protuberances and the ratiobetween the the total area of the top portions of the protuberances andthe area of the landing zone can be arbitrarily controlled only byadjusting the surface area of the substrate upon which the high energylight is to be irradiated, the output of such light, etc. The total areaof the top portions of the protuberances means the total area of theportions of the protuberances which are to actually contact a magnetichead slider.

In the first aspect of the present invention, as glass to constitute themagnetic disc substrate, chemically tempered glass such as soda limeglass, crystallized glass, alkaline-free glass, and alumino-silicateglass may be recited. Among them, chemically tempered glass andcrystallized glass are preferred from the standpoint of magnitude ofstrength.

Furthermore, crystallized glass has higher reliability in strength ascompared with chemically tempered glass because crystallized glasssuffers less variation in hardness and bending strength. Particularly inthe case where the thickness of the substrate is less than 0.5 mm, thecrystallized glass is extremely preferred in that strength is maintainedas desired. As such crystallized glass, Li₂ O--Si₂ --Al₂ O₃ --basedcrystallized glass is preferred. The composition and a productionprocess of novel and preferred crystallized glass will be explainedlater.

The term "surface portion" of the magnetic disc substrate means a zoneof the substrate up to the depth of at least 10 μm from the surfacethereof. According to the present invention, it is necessary that theabove metal ions are contained at least in this surface portion.

As the amount of the metal ions incorporated into the surface portion ofthe magnetic disc substrate increases, the protuberances formed by thehigh energy light becomes higher. In addition, there is a correlationamong the quantity, the scanning speed and throttled state of the highenergy light to be irradiated, the quantity of heat to be applied to thelight-irradiating point per unit area for a unit time period, and theheight of the protuberances. Thus, these parameters must beappropriately set.

The reason why the protuberances are formed by irradiating the highenergy light is not clear, but it is considered as follows. That is,locations of the magnetic disc substrate body where the metal ions arecontained in the surface portion absorb the irradiated light at a higherrate so that those locations reach a temperature sufficient to provokemass transfer. In the surface of the magnetic disc substrate body,working strain caused by the fine polishing remains. It is consideredthat the volumes of the above locations increase through release of thestrain by the heat caused by the high energy light, and that the masstransfer is done by the heat of the high energy light in such adirection that the area of the surface of the substrate polished flatand smooth decreases. Further, it may be considered that the crystallinephase contained in the substrate is transformed to an amorphous phase,and the volume of the relevant portion increases following thistransformation.

As the metal mentioned above, metal elements such as silver, copper (I),thallium, manganese, chromium, cobalt, iron, nickel, titanium, vanadium,cerium, and neodium are preferred. As the laser beam to be irradiatedupon the surface of the magnetic disc substrate body, YAG laser andargon ion laser beam are preferred. Particularly, YAG laser beam ispreferred. Further, the wavelength of the laser beam may be varieddepending upon the kind of the metal element incorporated into thesurface portion of the substrate.

In order to incorporate the metal element into at least the surfaceportion of the magnetic disc substrate body, the following two methodsmay be used.

Method I

The metal ions are incorporated into only a surface portion of themagnetic disc substrate. For this purpose, after the magnetic discsubstrate is finely polished, the magnetic disc substrate is immersedinto a melted salt containing the given metal ions, so that the metalions are diffused into the surface portion of the magnetic discsubstrate. Alternatively, the metal ions may be diffused into thesurface portion of the magnetic disc substrate by applying a pastecontaining the given metal ions upon the finely polished surface of themagnetic disc substrate and then heating the applied substrate.

In the above method, the silver ions, copper (I) ions and thallium ionsare particularly preferred, and among them silver ions are mostpreferred. As the melted salt for diffusing the metal ions into thesubstrate, silver nitrate, copper (I) chloride, and thallium nitrate maybe recited by way of example.

Method 2

The metal may be incorporated into the entire magnetic disc substratebody. In other words, an oxide containing metal ions may be incorporatedas one of glass components into a raw glass material to constitute themagnetic disc substrate. In this case, a glass preform for the substrateis produced through obtaining a glass material by mixing a metal aloneor a metal compound into a powdery glass starting material and meltingthe mixed powder.

According to the this magnetic disc substrate and its producing process,the above metal alone or the metal compound has only to be incorporatedinto the glass starting material during the producing step of the glasspreform in the method similar to a conventional magnetic discsubstrate-producing process, which is greatly advantageous from thestandpoint of the production. On the other hand, since the method bywhich the magnetic disc substrate is immersed into the melted salt needsthe immersion step, the number of the steps for the productionunfavorably increases by one. Furthermore, according to this method II,no variation is likely to occur in terms of the physical properties inevery portion of the magnetic disc substrate.

In the method II, the above metal oxide is preferably one or more kindsof metal oxides selected from the group consisting of manganese oxide,chromium oxide, cobalt oxide, iron oxide, nickel oxide, titanium oxide,vanadium oxide, cerium oxide, neodium oxide, silver oxide, copper oxide,thallium oxide. More preferably, the above metal oxide is one or morekinds of metal oxides selected from the group consisting of manganeseoxide, chromium oxide, cobalt oxide, iron oxide, nickel oxide, titaniumoxide, vanadium oxide, cerium oxide and neodium oxide. Further, as themetal oxide, one or more kinds of metal oxides selected from the groupconsisting of silver oxide, copper oxide, thallium oxide, iron oxide,chlonium oxide, and cobalt oxide may be used. As the metal compoundswhich can be incorporated into the raw glass material, the above metaloxides are preferred. In addition, hydroxides, carbonates, nitrates andphosphates of such metals may be used. Further, in case of chromiumoxide, dichromate compound may be used.

The addition amount of the metal or the metal compound to the raw glassmaterial is preferably in a range of 0.01 to 3.0 parts by weightrelative to 100 parts by weight of the other component(s) of the rawglass material when calculated in terms of the weight of the metaloxide. If the addition amount is less than 0.01 parts by weight, thehigh energy light having a given wavelength may not be effectivelyabsorbed. On the other hand, if the addition amount is more than 3.0parts by weight, evaporation may primarily occur at a location where thelight hits, so that a texture having an appropriate shape cannot beeasily formed.

The relationship between various above metal oxides and the wavelengthsof the high energy lights for effectively forming the textures is shownbelow.

                  TABLE 1                                                         ______________________________________                                              Wavelength                                                                 range of                                                                      high energy                                                                  Group light (nm) Kinds of metal oxides                                      ______________________________________                                         .sup. I                                                                            750 ˜ 1600                                                                           cobalt oxide, nickel oxide, vanadium                           oxide, iron oxide, copper oxide                                             .sup.  II 400 ˜ 750  Chromium oxide, cobalt oxide, neodium                                     oxide, manganese oxide, nickel                           oxide, copper oxide, silver oxide                                           III 200 ˜ 400  chromium oxide, vanadium oxide,                              titanium oxide, iron oxide,                                                 cerium oxide                                                              ______________________________________                                    

Among the above metal oxides, one kind of them may be incorporated intothe substrate. In this case, with respect to the substrate containingthe metal oxide belonging to Group I, the high energy light having thewavelength of 750 to 1600 nm is irradiated. With respect to thesubstrate containing the metal oxide belonging to Group II, the highenergy light having the wavelength of 400 to 750 nm is irradiated.Further, with respect to the substrate containing the metal oxidebelonging to Group III, the high energy light having the wavelength of200 to 400 nm is irradiated.

Two or more kinds of the metal oxides may be incorporated into thesubstrate. In this case, even if the metal oxides selected from thedifferent groups are simultaneously incorporated into the substrate andif the wavelength of the high energy light falls in the wavelength rangefor any one of these groups, the texture can be effectively formed byusing the high energy light having this wavelength. In this case, it ispreferable that the ratio of the metal oxide(s) belonging to each of thegroups is independently set at 0.01 to 3.0 parts by weight. In addition,the total amount of the metal oxides is preferably 0.01 to 10.0 parts byweight.

The height of the protuberances formed on the landing zone is preferablynot more than a half of a floating amount of a magnetic head slider.Thus, the height of the protuberances is more preferably not more than250 Å, more preferably not more than 200 Å. As the magnetic disc isrepeatedly used, the protuberances are abraded. Thus, the height of theprotuberances is preferably not less than 50 Å, and more preferably notless than 100 Å.

As the area of the top portions of the protuberances occupying thelanding zone increases, the frictional force becomes greater when themagnetic disc begins to turn. For this reason, it is preferable that theratio of the area of the top portions of the protuberances to the totalarea of the landing zone is preferably 5% or less. On the other hand,the ratio is preferably not less than 2% so that the abrasion of theprotuberances due to sliding between the magnetic head may be decreased.

According to the producing processes of the present invention, a numberof the protuberances may be continuously formed by intermittentlyirradiating the high energy light upon the finely polished magnetic discsubstrate while the substrate is being rotated in a constant directionrelative to the high energy light. In this case, the high energy lightsource may be rotated, or the magnetic disc substrate body may berotated, or both of them may be rotated. In that case, each protuberanceis of a planar arc shape extending in the circumferential direction ofthe magnetic disc substrate. Alternatively, the high energy light may beirradiated in a ring-shaped form upon the landing zone, or the highenergy light may be irradiated upon the landing zone through slits inthe form of a desired pattern. By so doing, the time required forforming the protuberances can be largely reduced.

In the crystallized glass particularly suitable for the production ofthe magnetic disc substrate according to the present invention, the maincrystal layer is occupied by a lithium disilicate (Li₂ O.SiO₂) phase anda β-spodumene (Li₂ O.Al₂ O₃.SiO₂) phase or a β-spodumene solid-solvedphase, and the rate of the SiO₂ crystal phase is not more than 2% byweight.

In order to produce the substrate made of such a crystallized glass, aglass preform having a composition of 65 to 85 wt % of SiO₂, 8 to 15 wt% of Li₂ O, 5 to 8 wt % of Al₂ O₃, and 1 to 5 wt % of P₂ O₅ is prepared.The crystallized glass is produced by heating the glass preform to athermally treating temperature of 820 to 950° C. Preferably, the centerline average surface roughness of the magnetic disc substrate made ofthe crystallized glass is preferably reduced to not more than 20 Å at atleast the surface on the magnetic recording side by finely polishingthat surface.

The present inventors have repeatedly made investigations on a Li₂O--SiO₂ --Al₂ O₃ based crystallized glass to constitute a magnetic disc,and an Li₂ O--SiO₂ phase and an SiO₂ phase are almost eliminated byspecifying the ratio of the starting materials and crystallizing theglass preform under the above-mentioned temperature condition so thatthe glass preform may be converted to an Li₂ O--Al₂ SiO₂ phase and aβ-spodumene (Li₂ O--Al₂ O₃ --4SiO₂) or a β-spodumene solid-solved phase.The main crystalline phase of this crystallized glass is composed mainlyof a lithium disilicate phase and the β-spodumene or the β-spodumenesolid-solved phase, and the ratio of the SiO₂ crystalline phase is notmore than 2 wt %.

The present inventors have succeeded in conspicuously reducing thecenter line surface roughness of the magnetically recording surface ofthe magnetic disc substrate made of such a crystallized glass down tonot more than 20 Å by finely polishing this surface. In addition, thetime period required for effecting the above fine polishing can beconspicuously reduced as compared the conventional magnetic discsubstrate made of the Li₂ O--SiO₂ --Al₂ O₃ based crystallized glass.

The reason why such function and effect are obtained is considered thatthe Li₂ O--₂ SiO₂ phase and the β-spodumene or the β-spodumenesolid-solved phase have almost the same hardness, that the maincrystalline phase is composed of these phases, and that a ceramic tissuecontaining almost no crystalline phase made of SiO₂ microscopicallyexhibits almost a monogeneous physical property upon grinding grains.

Al₂ O₃ is a component necessary for the formation of the β-spodumene orthe β-spodume solid-solved phase. If Al₂ O₃ is less than 5 wt % noβ-spodume is produced in the crystalline phase and the content of SiO₂crystalline phase exceeds 2 wt %, so that the center line surfaceroughness of the surface of the substrate after the polishing isdeteriorated.

SiO₂ is a fundamental component indispensable for obtaining acrystalline phase such as a lithium disilicate or the like. However, ifthe content of SiO₂ is less than 65 wt %, it is difficult to precipitatethe desired crystalline phase, whereas if it is more than 85 wt %, it isdifficult to melt the glass.

The present inventors thermally treated the glass preform as mentionedabove, and consequently discovered that 820° C. to 950° C. needs to beused as a crystallizing temperature. That is, it is conventionally knownthat the an Li₂ O--SiO₂ --Al₂ O₃ based glass is crystallized in a widerange of 700° C. to 950° C. However, according to the present invention,the solid solution phase composed preferably of 30-60 wt % of thelithium disilicate and 1-40 wt % of β-spodumene phase and β-spodumesolid solution phase were produced by crystallizing the glass preformhaving the above composition, while the ratio between the lithiumdisilicate and the β-spodumene and 62 -spodumene solid solution phasewas successfully not less than 1.0.

Further, the inventors discovered that in order to increase the strengthof the crystallized glass to the highest level, the crystallizingtemperature is preferably in a range of 820° C. to 920° C., particularlypreferably in a range of 820° C. to 900° C.

That is, if the heating temperature for the thermal treatment of theglass preform, namely, the crystallizing temperature, is in a range of700° C. to 750° C., 30 to 50% of the Li₂ O--SiO₂ phase and Li₂ O.2SiO₂phase occur, and a slight amount of the crystalline phase made of SiO₂is produced. As that time, as the heating temperature increases, boththe Li₂ O.SiO₂ phase and the Li₂ O.2SiO₂ phase increase. In this case,the center line average surface roughness can be reduced, but thestrength of the substrate is low. Thus, such a substrate cannot bepractically used.

If the crystallizing temperature is raised to around 800° C., the amountof the Li₂ O.SiO₂ phase rapidly decreases, whereas the amount of the Li₂O.2SiO₂ phase and the SiO₂ both rapidly increase. As a result, thecenter line average surface roughness increases, and also the timerequired for polishing conspicuously increases.

However, when the crystallizing temperature was raised to 820° C. theSiO₂ phase disappeared. At that time, the Li₂ O.2SiO₂ phase slightlyincreased. Further, it was found out that the amount of the β-spodumenephase rapidly began to be produced. That is, the crystallization of theAl₂ O₃ component first proceeded at this temperature at not less than820° C. so that the β-spodume phase (Li₂ O.Al₂ O₃.4SiO₂) or theβ-spodume solid solution were produced. Although the similar crystallinestructure is produced at the stage before the composition (Li₂ O.Al₂O₃.4SiO₂) is formed, the ratio among the Li₂ O, SiO₂ and Al₂ O₃ does notaccurately conform with that of Li₂ O.Al₂ O₃.4SiO₂. Thus, the abovecrystalline structure is called "β-spodume solid solution".

In the range of 820° C. to 900° C., the lithium disilicate phase, theβ-spodume phase or β-spodumene solid solution phase gradually increase.In this temperature range, the average crystalline particle diameter isnot more than 1.0 μ, and the strength of the substrate can be kept at anextremely high level. If the temperature exceeds 900° C., the averagecrystalline particle size of the glass ceramic exceeds 1.0 μm, althoughthe crystalline phase does not largely change. Consequently, thestrength of the substrate begins to decrease. If the temperature exceeds920° C., the strength of the substrate tends to further decrease.

Furthermore, it was clarified that a problem occurs if the β-spodumephase increases as compared with the original lithium disilicate in thecase of the substrate of the present invention. That is, if theβ-spodumene particles proceed to grow such that the weight ratio of thelithium disilicate phase/(the β-spodumene phase plus the β-spodume solidsolution in total) may be decreased to less than 1.0, the mechanicalstrength of the substrate for the magnetic disc decreases.

From the above, it was found that the above weight ratio is morepreferably not less than 1.0.

If the amount of the Al₂ O₃ in the glass preform exceeds 8 wt %, theβ-spodume particles proceed to grow, so that the strength of thesubstrate for the magnetic disc decreases. Therefore, it is preferablethat the amount of the Al₂ O₃ is not more than 8 wt %.

Further, when the glass preform is heated in the above productionprocess, the production of the crystalline nuclei is preferably promotedby controlling the heating rate at least in a temperature range of notless than 500° C. to 50° C./h to 300° C./h. In addition, the productionof the crystalline nuclei is more preferably promoted by holding theglass preform at least in a temperature range of 500° C. to 580° C. for1 to 4 hours.

One or more other components may be incorporated into the crystallizedglass of this type. As a nuclei-forming agent other than P₂ O₅, metaloxides such as TiO₂, ZrO₂ and SnO₂, a metal such as platinum and afluoride may selectively be incorporated singly or in a mixed state oftwo or more kinds of them. Further, 0 to 7 wt % of K₂ O may beincorporated. This functions to reduce the melting temperature and theshaping temperature of the glass and also functions to preventdevitrification of the glass during the shaping. In order to exhibitssuch functions, the content of K₂ O is preferably not less than 2 wt %.Further, if the content is more than 7 wt %, the strength of the glassceramic tends to decrease. One or both of As₂ O₃ and Sb₂ O₃ may beincorporated in a total amount of 0 to 2 wt %. These compounds functionas a transparency-imparting agent on melting the glass.

Moreover, 0 to 3 wt % of a B₂ O₃ component, 0 to 3 wt % of a CaOcomponent, 0 to 3 wt % of a SrO, and 0 to 3 wt % of BaO may beincorporated. Substantially no MgO component is preferably incorporated.

In the production of the glass preform, starting materials containingabove metal atoms are mixed according to the above weight ratio, and theresulting mixture is melted. As the starting materials, oxides,carbonates, nitrates, phosphates, and hydroxides of the above metalatoms may be recited by way of example. In addition, an atmosphericatmosphere, a steam atmosphere and a pressurizing atmosphere may beselectively employed as an atmosphere to crystallize the glass preformunder heating.

In order to finely polish the magnetic disc substrate, a known finepolishing such as so-called lapping and polishing may be used.

The crystallized glass itself and the production process thereofaccording to the second aspect of the present invention will besuccessively explained. The present inventors repeatedly examined theLi₂ O--Al₂ O₃ --SiO₂ based crystallized glass, but found it difficult tosolve the above problems. That is, it was made clear that theβ-spoudumene is ordinarily precipitated in the crystallized glass ofthis type (typically described in Japanese patent application No.1-174895), and aggregated particles may be formed through theaggregation of crystals depending upon the kind of an additive in thecomposition of the starting materials and the heating temperature forthe crystallization, and the center line average surface roughness isfurther increased by such aggregated particles.

For example, JP-A-6-329,440 describes a process for controlling thesurface roughness of the Li₂ O--Al₂ O₃ --SiO₂ based crystallized glass.However, the center line average surface roughness cannot be reduced toa level of not more than 10 Å by this process.

The present inventors crystallized a raw glass having the composition ofLi₂ O--Al₂ O₃ --SiO₂ which contains 65 to 85 wt % of SiO₂, 8 to 15 wt %of Li₂ O, 2 to 8 wt % of Al₂ O₃, 1 to 5 wt % of P₂ O₅ and 1 to 10 wt %of ZrO₂ at various temperatures in the state that ZrO₂ was contained inthe raw glass. The inventors surprisingly discovered that in a givencrystallizing temperature range, the aggregation of the secondaryparticles disappeared and fine crystals were extremely uniformlydispersed among the glass phases. As a result, it was discovered thatthe center line average surface roughness after the finely polishing thecrystallized glass was extremely decreased, and that it could be reducedto Ra of not more than 10 Å. The present inventors reached theirinvention.

If no ZrO₂ is added, an eucryptite phase (Li₂ O.Al₂ O₃.2SiO₂) phase andthe spodumene (Li₂ O.Al₂ O₃.4SiO₂) phase are predominant in the crystal.When 0.5 wt % of ZrO₂ is added, a still considerable amount of theeucryptite phase and the spodumene phase remained, although slightlydecreased, and some aggregated particles remained. When not less than1.0 wt % of ZrO₂ was added, both of the eucryptite phase and thespodumene phase were remarkably reduced, and the aggregation to thesecondary aggregates was not observed through a microscope. It was alsoseen that the main crystalline phase was lithium disilicate (Li₂O.2SiO₂). In addition, it was confirmed that since the crystallizationproceeded, the crystallized glass had strength large enough to be usedas a magnetic disc substrate.

When the intensity of the X-ray diffraction peak of lithium disilicateis taken as 100, the sum of the intensity of the eucryptite phase andthat of the spodumene phase is preferably not more than 50, and morepreferably not more than 40.

In order to crystallize the glass preform, the maximum temperature inthe crystallization step needed to be 680° C. to 700° C. When themaximum temperature was more than 770° C., the eucryptite, etc. werestill produced, and the center line average surface roughness (Ra) afterthe fine polishing increased. From this point, it is more preferable toset the maximum temperature at not more than 760° C. Strength of thecrystallized glass was increased by setting the maximum temperature atnot less than 680° C. From this point of view, setting at not less than700° C. is more preferable.

The crystallized glass according to the present invention may containthe Al₂ O₃ phase, the β-cristobalite phase and Li₂ O.SiO₂ phase. Whenthe intensity of the X-ray diffraction peak of lithium disilicate istaken as 100, the intensity of the X-ray diffraction peak of the Al₂ O₃phase is preferably not more than 50, and the intensity of the X-raydiffraction peak of the β-cristobalite is preferably not more than 50,whereas the intensity of the X-ray diffraction peak of the LiO₂.SiO₂phase is preferably not more than 70. The lower limit of each of thesephases is zero. The crystallized glass according to the presentinvention does not substantially contain α-quartz, that is α-quartz isnot detected by X-ray diffraction or its peak is not more than 5.Further, the crystallinity of the crystallized glass is preferably notless than 60%.

In the composition of the glass preform, SiO₂ is a fundamental componentindispensable for obtaining lithium disilicate. If the content of SiO₂is less than 65 wt %, it is difficult of precipitate the desiredcrystalline phases, whereas if it is more than 85%, it is difficult tomelt glass.

If the Al₂ O₃ component is more than 8 wt % in the glass preform, theproduction amount of the eucryptite phase tends to be excessive, so thatstrength of the magnetic disc substrate decreases and the center lineaverage roughness increases.

As mentioned above, the content of ZrO₂ needs to be not less than 1 wt%. If this content is not less than 2 wt %, the center line averagesurface roughness can be further decreased. If the content of ZrO₂ ismore than 10 wt %, the melting temperature of glass increases, whichmakes industrial handling difficult. From this point of view, it ispreferable to set the content of ZrO₂ at not more than 8 wt %, and morepreferable to set it at not more than 4 wt %.

Other component(s) may be contained in the crystallized glass accordingto the present invention. TiO₂, SnO₂ and a fluoride of a noble metalsuch as platinum can be incorporated singly or in a mixed state of twoor more kinds of them.

Further, 0 to 7 wt % of K₂ O may be incorporated. This functions tolower the melting and shaping temperatures of glass and to preventdevitrification of glass during shaping. In order to exhibit the abovefunctions, the content of K₂ O is preferably set at not less than 2 wt%. If this content exceeds 7 wt %, strength of the crystallized glasstends to lower. One or both of As₂ O₃ and Sb₂ O₃ may be contained in atotal amount of 0 to 2 wt %. They are refining agents. In addition, 0 to3 wt % of B₂ O₃ component, 0 to 3 wt % of CaO component, 0 to 3 wt % ofSrO and 0 to 3 wt % of BaO may be incorporated.

However, as mentioned above, substantially no MgO component is containedin the crystallized glass. That substantially no MgO component iscontained in the crystallized glass means that MgO component originatingas inevitable impurity contained in other powdery starting components isnot excluded.

The glass preform is produced by mixing starting materials containingthe above metal atoms at the above weight ratio and melting theresulting mixture. As such starting materials, oxides, carbonates,nitrates, phosphate, and hydroxides of those metallic atoms may berecited by way of example. Further, as an atmosphere for thecrystallization of the glass preform under heating, open air atmosphere,reducing atmosphere, steam atmosphere, pressurizing atmosphere or thelike may be selected.

When the glass preform is heated in the above producing process, it ispreferable to produce crystal nuclei under condition that thetemperature range is at least not less than 500° C. and the heating rateis 50 to 300° C./hour. Moreover, it is preferable to produce the crystalnuclei by holding the glass at least in a temperature range of 500° C.to 580° C. for 1 to 4 hours.

In order to finely polish a member made of the above crystallized glasswith grinding grains, a magnetic disc substrate may be produced by aknown fine polishing such as lapping or polishing. In addition, an undertreating layer, a magnetic film, a protective film or the like may beformed on a main plane of the magnetic disc substrate according to thepresent invention, and further a lubricant may be coated upon theprotective layer.

Besides the reading/writing zone, a landing zone may be provided at themagnetic disc substrate made of the crystallized glass according to thepresent invention. The center line average surface roughness (Ra) of thereading/writing zone is preferably 5 to 10 Å, whereas the maximum heightof the reading/writing zone is not more 100 Å.

Next, the present inventors contrived that when a magnetic discsubstrate body was to be produced from the crystallized glass accordingto the second aspect, a metallic element absorbing light wasincorporated into at least a surface portion of the magnetic discsubstrate body and a texture was formed by irradiating high energy lightupon its surfacer and the inventors actually made experiments.Consequently, the inventors discovered that a texture having acontrolled height could be formed on an extremely flat and smoothsurface finely polished with the center line average surface roughnessof not more than 10 Å.

The process for incorporating the metallic element absorbing light in atleast the surface portion of the magnetic disc substrate body may be thesame as the two processes as explained in connection with the firstaspect of the present invention mentioned above. In particular, theabove metallic element was favorably incorporated into the crystallizedglass in the form of an oxide.

The present inventors proceeded with investigations, and discovered thatif the above oxide is other than chromium oxide, a texture having anappropriate height can be formed at high productivity by setting thetotal addition amount of the oxide(s) at not less than 0.01 parts byweight. Further, it was seen that as the addition amount of the oxide(s)was increased, the intensity of the peak of β-eucryptite phase tended toincrease. If this intensity is increased beyond a certain level, itbecomes difficult to form a flat and smooth surface having the centerline average surface roughness of not more than 10 Å after the finepolishing. From this point of view, when the total addition amount ofthe oxide(s) is not more than 3 parts by weight, particularly theproduction of the β-eucryptite phase can be suppressed, and the texturescan be mass-produced while realizing the flat and smooth polishedsurfaces.

For example, when iron oxide, manganese oxide and/or cobalt oxide wereadded, the intensity of the peak (2θ=26.1°) of the β-eucryptite phasewas not less than 50 when the intensity of the peak (2θ=24.8°) oflithium disilicate was taken as 100, if the total or single additionamount of these oxide exceeded 3 parts by weight. Consequently, thecenter line average surface roughness could not be suppressed to notmore than 10 Å.

However, if chromium oxide among the above oxides is added, it functionsto suppress the production of the β-eucryptite phase. Therefore, if atleast chromium oxide is included in the additives, the addition amountof chromium oxide is preferably not less than 0.01 parts by weight butnot more than 10 parts by weight. In particular, if chromium oxide isused together with an oxide other than chromium oxide, it is preferablethat the addition amount of chromium oxide is not less than 0.5 parts byweight but not more than 10 parts by weight and the total additionamount is not less than 3 parts by weight but not more than 10 parts byweight.

As the metal capable of absorbing light and to be incorporated into themagnetic disc substrate body according to the present invention,transition metal elements are preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a plane view showing an embodiment of a magnetic discsubstrate 1;

FIG. 1(b) is a plane view showing an embodiment of a magnetic disc 10;

FIG. 2(a) is a plane view schematically showing a plane shape of atexture formed on a landing zone 3;

FIG. 2(b) is a partially enlarged view of FIG. 2(a);

FIG. 3 is a graph showing the relationship between a time period forimmersing a magnetic disc substrate into a molten salt of AgNO₃ and theheight of protuberances formed by irradiation of laser beam;

FIG. 4 is a graph showing results obtained by measurement of thesectional shapes of protuberances produced by laser processing with asurface roughness meter;

FIG. 5 is a graph showing the relationship between the center lineaverage surface roughness (Ra) and the crystallizing temperature in thecrystallized glasses having different compositions in the case of thesecond aspect of the present invention;

FIG. 6 is a graph showing the relationship between the diffractionintensity of the eucryptite phase and the crystallizing temperature forcrystallized glasses having different compositions;

FIG. 7 is a scanning type electron electronic microscope photographshowing a ceramic tissue of a finely polished surface obtained bypolishing a crystallized glass which was produced by using a rawmaterial of a composition C and setting a crystallizing temperature at750° C.;

FIG. 8 is a scanning type electron electronic microscope photographshowing a ceramic tissue of a finely polished surface obtained bypolishing a crystallized glass which was produced by using a rawmaterial of the composition C and setting a crystallizing temperature at790° C.;

FIG. 9 is a scanning type electron electronic microscope photographshowing a ceramic tissue of a finely polished surface obtained bypolishing a crystallized glass which was produced by using a rawmaterial of a composition A and setting a crystallizing temperature at750° C.; and

FIG. 10 is a scanning type electron electronic microscope photographshowing a ceramic tissue of a finely polished surface obtained bypolishing a crystallized glass which was produced by using a rawmaterial of a composition F and setting a crystallizing temperature at750° C.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1(a), a reading/writing zone 2 and a landing zone 3 maybe separately provided on a substrate 1 for a magnetic disc according tothe present invention. The height (Rp) of the flatness of the landingzone 3 is preferably in a range of 50 to 200 Å. Furthermore, it ispreferable that a spacer-laminating section 4 is provided inside thelanding zone 3, and that the center line average surface roughness ofthe spacer-laminating section 4 is not more than 3 nm. A round hole 5 isformed inside the spacer-laminating section 4.

In the magnetic disc substrate according to the present invention, atleast a magnetic film needs to be formed on a main face of the abovesubstrate. Preferably, as shown in FIG. 1(b), a magnetic disc 10 may beproduced by forming an undertreating layer 6, a magnetic film 7, aprotective film 8 and a lubricating film 9 successively on the surfaceof the magnetic disc substrate 1.

In the following, specific experimental results will be explained.

EXAMPLE 1

Production of a Glass Preform for a Magnetic Disc Substrate

Powdery compounds containing respective metals were mixed to give 76.5wt % of SiO₂, 7.1 wt % of Al₂ O₃, 11.4 wt % of Li₂ O, 2.9 wt % of K₂ O,1.9 wt % of P₂ O₅ and 0.2 wt % of Sb₂ O₃. The mixture was melted byheating at 1350° C. for 20 hours. The melt was shaped, and a glasspreform having a circular shape was obtained by gradual cooling.

Production of a Magnetic Disc Substrate

The resulting glass preform was heated at a heating rate of 50° C./h inatmosphere, and thermally treated at 850° C. in the atmosphere for 4hours. Then, the glass was cooled down to room temperature at a coolingrate of 50° C./h, thereby obtaining a substrate material made of acrystallized glass. This substrate material had a round discoidal shapeof 65 mm in an outer diameter and 20 mm in an inner diameter.

Crystalline phases of this substrate material were identified, and theweight ratio of the crystalline phases was determined by measurement.This determination was effected by a X-ray diffraction. As a result, thepercentage of lithium disilicate was 48 wt %, and that of the β-spodumephase 19 wt %.

Opposite main surfaces of the substrate material were polished flat in athickness of 0.75 mm and a flatness of 8 μm by using a diamond grindingstone. Then, a finely polished body having a thickness of 0.64 mm wasobtained by lapping the opposed polished main surfaces of the substratematerial with GC grinding grains. Thereafter, the resulting substratematerial was finish polished to a thickness of 0.635 mm by usinggrinding grains of cerium oxide. The resulting substrate material hadthe center line average surface roughness of 20 Å and a flatness of notmore than 5 μm. Introduction of silver ions into a surface portion ofthe magnetic disc substrate.

The above magnetic disc substrate was immersed in isopropyl alcohol at25° C. for 4 minutes, and washed. Then, the magnetic disc substrate wasimmersed into a melted 97% AgNO₃ liquid (temperature: 250° C.), therebyincorporating the silver ions into the surface portion of the magneticdisc substrate.

Formation of a Texture in the Substrate Body by Irradiating Laser Beam

A YAG laser beam (wavelength 1.06 μm, output 50 W) was irradiated uponthe surface of the magnetic disc substrate under rotation. The rotatingspeed of the magnetic disc substrate was 60 mm/sec. Thereby, a number ofprotuberances having a planar shape as shown in FIGS. 2(a) and 2(b) wereformed. More specifically, a number of the slender protuberances 11 werecircumferentially formed in the landing zone 3 such that theprotuberances 11 were arranged in plural stages as viewed in a radialdirection. The surface state of the protuberances was measured by asurface roughness meter.

The relationship between the height of the protuberances and the timeperiod during which the magnetic disc substrate was immersed into themelt is shown in FIG. 3. Further, the sectional shapes of theprotruberances formed on the magnetic disc substrate when the immersiontime period was set at 60 seconds were measured by the surface roughnessmeter, and results are shown in FIG. 4.

As is seen from the above results, it was clarified that when themagnetic disc substrate was immersed into the melt at 250° C. for 30 to60 seconds, the protuberances having heights of 100 to 200 Å could beformed under the above laser beam-irradiating condition.

EXAMPLE 2

As in Example 1, opposite surfaces of a magnetic disc substrate werefinish polished, and than the substrate was immersed in a 97% AgNO₃ meltheld at 250° C. for 45 seconds, thereby introducing Ag ions into asurface portion of the magnetic disc substrate. A laser beam wasirradiated upon a landing zone of the magnetic disc substrate under theabove laser-irradiating condition according to a pattern shown in FIG.2(a) and 2(b). As a result, protuberances were formed at the laserbeam-irradiated portion of the substrate at the average height of 170 Å.

In FIG. 2(b), the protuberances had the average radial width "c" of 0.05mm and the circumferential length "a" of 0.065 mm. The pitch "b" betweenthe circumferentially adjacent protuberances 11 was 0.8 mm, and thepitch "d" between the radially adjacent protuberances is 0.2 mm. Thetime period required for the formation of the protuberances was only 10seconds per one magnetic disc substrate 1. As shown in FIG. 1(b),various layers were formed on the magnetic disc substrate body as shownin FIG. 1(b), thereby producing a magnetic disc 10 for CSS(contact-start stopping) testing. That is, a 150 nm thick sputtered filmof chromium was formed as a under layer 6 on the surface of the magneticdisc substrate 1, and a Co--Ta--Cr magnetic film 7 was formed in athickness of 60 nm on the surface of the under layer 6. Then, aprotective film 8 made of a carbon film was formed in a thickness of 30nm by sputtering. Then, a lubricant layer 9 was formed on the surface ofthe protective layer 8 by coating.

When the magnetic disc 10 was subjected to the CSS testing, a 50% sizethin film magnetic head was used as a magnetic head slider, and the CSStesting was effected in the landing zone at the number of revolutions of4500 rpm under a gram load of 3.5 g. The result was good in that thecoefficient of friction between the magnetic head and the magnetic discwas 0.3 even after 50,000 times CSS testing.

EXAMPLE 3

Samples were produced according to Experimental Nos. shown in Table 2similarly as in Example 1.

Production of a Glass Preform for a Magnetic Disc Substrate

Powdery compounds containing respective metals were mixed to give 76.5wt % of SiO₂, 7.1 wt % of Al₂ O₃, 11.4 wt % of Li₂ O, 2.9 wt % of K₂ O,1.9 wt % of P₂ O₅ and 0.2 wt % of Sb₂ O₃. At that time, an additive wasincorporated into the mixed powder in a given parts by weight relativeto 100 parts by weight of the latter as shown in Table 2. InExperimental No. 3-3, no metal oxide additive was incorporated. Theresulting mixture was charged into a platinum crucible, and melted byheating it at 1450° C. for 15 hours. The melt was poured into a mold,and gradually cooled. A glass preform having a round planar shape wasobtained by cutting the molding.

Production of a Magnetic Disc Substrate

The resulting glass preform was heated at a heating rate of 50° C./h inatmosphere, and thermally treated at 850° C. in the atmosphere for 6hours. Then, the glass was cooled down to room temperature at a coolingrate of 50° C./h, thereby obtaining a substrate material made of acrystallized glass. This substrate material had a round discoidal shapeof 65 mm in an outer diameter and 20 mm in an inner diameter.

Crystalline phases of this substrate material were identified, and theweight ratio of the crystalline phases was determined by measurement.This determination was effected based on a X-ray diffraction. As aresult, the percentage of the lithium disilicate was 48 wt %, and thatof the β-spodumene phase 19 wt %.

Opposite main surfaces of the substrate material were polished flat in athickness of 0.75 mm and a flatness of 8 μm by using a diamond grindingstone. Then, a finely polished body having a thickness of 0.64 mm wasobtained by lapping the opposed polished main surfaces of the substratematerial with GC grinding grains. Thereafter, the resulting substratematerial was finish polished to a thickness of 0.635 mm by usinggrinding grains of cerium oxide. The resulting substrate material hadthe center line average surface roughness Ra of 10 Å and a flatness ofnot more than 5 μm.

Formation of a Texture by Irradiating Laser Beam

A YAG laser beam was irradiated upon the surface of the magnetic discsubstrate under rotation. The rotating speed of the magnetic discsubstrate was 60 mm/sec. Thereby, a number of protuberances having aplanar shape as shown in FIGS. 2(a) and 2(b) were formed. Morespecifically, a number of the slender protuberances 11 werecircumferentially formed in the landing zone 3 such that theprotuberances 11 were arranged in plural stages as viewed in a radialdirection. The height and the surface state of the protuberances weremeasured by the surface roughness meter.

                  TABLE 2                                                         ______________________________________                                        Experi-                                                                             Wavelength of                 Height of                                   mental    laser beam    Addition ratio    protuberances                       No.        (nm)     Additive (parts by weight) (Å)                      ______________________________________                                        3-1   1064       NiO      3.0       150                                         3-2        1064        CuO          3.0             180                       3-3         533        none         --                 0                      3-4         533        MnO.sub.2         0.005            20                  3-5         533        MnO.sub.2         0.01             50                  3-6         533        MnO.sub.2         2.0             150                  3-7         533        MnO.sub.2         3.0             200                  3-8         533        MnO.sub.2         5.0             800                ______________________________________                                    

In Experimental Nos. 3-1 and 3-2 falling in the scope of the presentinvention, the laser beam having a wavelength of 1064 nm was used, and3.0 parts by weight of NiO or CuO was incorporated. As a result,protuberances each having a preferred pointed shape of 150 Å or 180 Åcould be formed. In Experimental No. 3-3 falling outside the scope ofthe invention, no protuberances could be formed. In Experimental Nos.3-4 to 3-8, MnO₂ was incorporated into the substrate body, while itsaddition amount was varied in various ways. As a result, protuberancescould be formed in these experimental samples. However, since thepreferred height of the protuberances in the texture is ordinarily 50 to200 nm the addition amount of MnO₂ is preferably 0.01 to 3.0 parts byweight.

EXAMPLE 4

Samples were produced according to Experimental Nos. in Table 3similarly as in Example 3, provided that the kind and the additionamount of the metal compound were varied as shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Experi-                                                                             Wavelength of                 Height of                                   mental   laser beam     Addition ratio     protuberances                      No.        (nm)      Additive   (parts by weight) (Å)                   ______________________________________                                        4-1   533        CoO      1.0       60                                          4-2        533           Nd.sub.2 O.sub.3          2.0              100       4-3        266           TiO.sub.2           1.0               80                                                4-4        266           V.sub.2                                             O.sub.5           1.0                                                         170                                         4-5        266         K.sub.2 Cr.sub.2 O.sub.7          1.0                                                      160                                       4-6        266           Fe.sub.2 O.sub.3          1.0              120       4-7        266           CeO.sub.2           1.0              180           ______________________________________                                    

In Experimental Nos. 4-1 and 4-2 falling in the scope of the presentinvention, a laser beam having a wavelength of 533 nm was used, and 1.0parts by weight of CoO or 2.0 parts by weight of Nd₂ O₃ wasincorporated. As a result, protuberances having a preferred pointedshape of 60 Å or 100 Å could be formed. In Experimental Nos. 4-3 to 3-7,a laser beam having a wavelength of 266 nm was used, and 1.0 parts byweight of TiO₂, V₂ O₅, K₂ Cr₂ O₇, Fe₂ O₃ or CeO₂ was incorporated intothe substrate. As a result, protuberances having heights from 80 to 170Å and preferred pointed tips could be formed in these Experimentalsamples.

EXAMPLE 5

Samples were produced according to Experimental Nos. in Table 4similarly as in Example 3, provided that the kind and the additionamount of the metal compounds were varied as shown in Table 4. In eachExperimental sample, three kinds of the metal oxides were added.

                  TABLE 4                                                         ______________________________________                                        Experi-                                                                              Wavelength of              Height of                                     mental    laser beam        Additive & addition ratio      protuberances      No.        (nm)                                         (parts by                                             weight)       (Å)                       ______________________________________                                        5-1    1064       MnO.sub.2 :1.51, CoO:0.13                                                                     50                                               K.sub.2 Cr.sub.2 O.sub.7 :1.88                                             5-2                  533 MnO.sub.2 :1.51, CoO:0.13          200                                                  K.sub.2 Cr.sub.2 O.sub.7 :1.88                                              5-3                  266 MnO.sub.2                                           :1.51, CoO:0.13          160                        K.sub.2 Cr.sub.2 O.sub.7 :1.88                                          5-4                  533 CoO:0.05, TiO.sub.2 : 0.50           50                                                  Fe.sub.2 O.sub.3 :0.70                    5-5 533 MnO.sub.2 :0.54, CoO:0.05 110                                                            Fe.sub.2 O.sub.3 :0.70                                   ______________________________________                                    

In Experimental Nos. 5-1 and 5-3 falling in the scope of the presentinvention, MnO₂, CoO, and K₂ Cr₂ O₇ were incorporated, and thewavelength of the laser beam was varied among 1064 nm, 533 nm and 266nm, respectively. As a result, protuberances each having a preferredpointed shape of 50 Å or 200 Å could be formed in each case,respectively.

In Test Run Nos. 5-4 and 5-5, CoO₂, Fe₂ O₃, and TiO₂ or MnO₂ wereincorporated into the substrate body, and a laser beam having awavelength of 533 nm was used. As a result, protuberances having heightsof 50 and 110 Å, respectively and preferred pointed tips could be formedin these Test Runs.

In the following, experimental results regarding the second aspect ofthe present invention will be described.

A raw material of each composition was prepared to give a ratio inoxides shown in Table 5. The ratio shown in Table 5 means weightpercentages of metal oxides in a glass preform.

                                      TABLE 5                                     __________________________________________________________________________           Composi-                                                                           Composi-                                                                           Composi-                                                                           Composi-                                                                           Composi-                                                                           Composi-                                        Ingredients  tion A   tion B   tion C  tion D   tion E   tion F             __________________________________________________________________________    SiO.sub.2                                                                            76.1 75.7 74.6 76.0 76.0 71.0                                            Li.sub.2 O       11.8     11.7     11.6     10.0    10.0     10.0                                            Al.sub.2 O.sub.3       7.1      7.1                                          7.0      7.0     7.0      6.0                   K.sub.2 O         2.8      2.8      2.7      2.8     2.7      6.0                                            ZrO.sub.2        0.0      0.5      2.0                                          2.0     2.0     1.0                          P.sub.2 O.sub.5        2.0      2.0      2.0      2.0     2.0      2.0                                       Sb.sub.2 O.sub.3       0.2      0.2                                          0.2      0.2     0.2      0.5                   Fe.sub.2 O.sub.3       0.0      0.0      0.0      0.0     0.1      0.0                                       MgO         0.0      0.0      0.0                                            0.0     0.0      3.5                          __________________________________________________________________________

Producing Examples from Composition C

Compounds containing respective metals were mixed to give a weight ratioof oxides shown in Composition C of Table 5, and the resulting mixturewas melted by heating at 1400° C. The melt was molded, and graduallycooled, thereby obtaining a glass preform. Crystal nuclei were formed byholding the resulting glass preform at 520° C. in an nitrogen atmospherefor one hour. The glass was heated at a heating rate of 100° C./hour,and held at a given crystallizing temperature for 4hours, followed bycooling it to room temperature. Thereby, a crystallized glass wasobtained.

An experimental sample, 15 mm long, 15 mm wide and 0.8 mm thick, was cutout from each crystallized glass, and opposite surfaces of thisexperimental sample were finish polished by using cerium oxide grindingparticles having the average particle diameter of 1.5 μm on anopposite-face polishing table bonded with polyurethane pads. The centerline average surface roughness (Ra) of the finish polished surfaces wasmeasured by using a surface roughness meter attached with a touchingneedle having a diameter of 0.5 μm. Measurement results are shown inTable 6. Crystalline phases in the crystallized glass were identified bythe X-ray diffraction method, and the intensity of the peak of eachcrystalline phase was measured. A relative ratio of the intensity of thepeak of each crystalline phase is shown in Table 6, while the intensityof the peak of lithium disilicate (Li₂ O.2SiO₂) is taken as 100. Therelationship between the center line average surface roughness (Ra) andthe crystallizing temperature is shown in FIG. 5, and the relationshipbetween the intensity (absolute value) of diffraction of the eucryptitephase and the crystallizing temperature is shown in FIG. 6.

                                      TABLE 6                                     __________________________________________________________________________    Composition C                                                                 Crystal-                                                                        lizing                                                                        temperature Ra Li.sub.2 O · AL.sub.2 O.sub.3 ·                                           Li.sub.2 O · Al.sub.2 O.sub.3                                        ·    β-cristo-                     (° C.) (A) 2SiO.sub.2 · 4SiO.sub.2 Al.sub.2 O.sub.3                                        balite Li.sub.2 O · SiO.sub.2         __________________________________________________________________________    870   27                                                                              0      67     0   0    0                                                850      23     65          0        0      0         0                       830      21     95          0        0      0         0                       810      22     98          0        0      0         0                       790      13     51          0        0      0         0                       770       9     13          0        12     0         0                       750               9 7          0        13     0         0                    730       9     20          0        28     30        50                    __________________________________________________________________________

Producing Examples of Composition A

Each experimental sample was produced in the same manner as above, andRa and the relative ratio in the intensity of the peak of the respectivecrystalline phase were measured with respect to the each experimentalsample. Results are shown in Table 7. The relationship between thecenter line average surface roughness (Ra) and the crystallizingtemperature is shown in FIG. 5, and the relationship between theintensity of diffraction of the eucryptite phase and the crystallizingtemperature is shown in FIG. 6.

                                      TABLE 7                                     __________________________________________________________________________    Composition A                                                                 Crystal-                                                                        lizing                                                                        temperature Ra Li.sub.2 O · AL.sub.2 O.sub.3 ·                                           Li.sub.2 O · Al.sub.2 O.sub.3                                        ·    β-cristo-                     (° C.) (A) 2SiO.sub.2 · 4SiO.sub.2 Al.sub.2 O.sub.3                                        balite Li.sub.2 O · SiO.sub.2         __________________________________________________________________________    870   25                                                                              0      55     0   0    0                                                850       23      0           56        0       0          0                  830       21      41          39        0       0          0                  810       21      34          40        0       0          0                  790       20      82           0        0       0          0                  770       19      83           0        0       0          0                  750       16      79           0        0       0          8                  730       15      58           0        0                  59 67                                            700       10      35           0        0                                          40        85                             __________________________________________________________________________

Producing Examples of Composition B

Each experimental sample was produced in the same manner as above, andRa and the relative ratio in the intensity of the peak of the respectivecrystalline phases were measured with respect to the each experimentalsample. Results are shown in Table 8. The relationship between thecenter line average surface roughness (Ra) and the crystallizingtemperature is shown in FIG. 5, and the relationship between theintensity of diffraction of the eucryptite phase and the crystallizingtemperature is shown in FIG. 6.

                                      TABLE 8                                     __________________________________________________________________________    Composition B                                                                 Crystal-                                                                        lizing                                                                        temperature Ra Li.sub.2 O · AL.sub.2 O.sub.3 ·                                           Li.sub.2 O · Al.sub.2 O.sub.3                                        ·    β-cristo-                     (° C.) (A) 2SiO.sub.2 · 4SiO.sub.2 Al.sub.2 O.sub.3                                        balite Li.sub.2 O · SiO.sub.2         __________________________________________________________________________    870   26                                                                              0      62     0   0    0                                                850       22      0          60        0       0         0                    830       18      0          56        0       0         0                    810       16      0          53        0       0         0                    790       17      77         0         0       0         0                    770       16      69         0         0       0         0                    750       12      51         0         0       0         0                    730       12      48         0         0       25        48                 __________________________________________________________________________

Producing Examples of Compositions D and E

Each experimental sample was produced in the same manner as above, andRa and the relative ratio in the intensity of the peak of the respectivecrystalline phases were measured with respect to the each experimentalsample. Results are shown in Table 9.

                                      TABLE 9                                     __________________________________________________________________________         Crystal-                                                                    lizing                                                                        tempera-                                                                     Composi- ture Ra Li.sub.2 O · AL.sub.2 O.sub.3 ·                                             Li.sub.2 O · Al.sub.2 O.sub.3                                        · Al.sub.2 O.sub.3 β-cris                                       to- Li.sub.2 O ·                    tion (° C.) (A) 2SiO.sub.2 4SiO.sub.2 Al.sub.2 O.sub.3 balite                                           SiO.sub.2                                  __________________________________________________________________________    D    790  27                                                                              234    0      0   0    0                                            D      770     10        48         0       15      16      0                 D      750      9        22         0       22      20      0                 D      730      7        14         0       19      0       24                E      790     28      203          0       0       0       0                 E      770                9 40         0       17      0       0                                                E      750      8        14         0                                              20      0       0                      E      730      7        0          0       18      0       0               __________________________________________________________________________

Evaluation of Crystallized Glasses Using Compositions A, B, C, D and E

No ZrO₂ is added in Composition A. In this case, although the centerline average surface roughness is gradually decreased by lowering thecrystallizing temperature, it is about 15 Å even at the crystallizingtemperature of 730° C. The intensity of diffraction of the eucryptitephase is large.

In Composition B, 0.5 wt % of ZrO₂ is added. However, no conspicuousdifferences are observed with respect to the center line average surfaceroughness and the crystalline phases such as eucryptite phase ascompared with the cases using Composition A.

On the other hand, in Composition C using a raw material in which 2.0 wt% of ZrO₂ was added, the center line average surface roughness near thecrystallizing temperature of 770° C. conspicuously decreased, andconsequently the center line average surface roughness of not more than10 Å was obtained. In addition, reduction in strength of thecrystallizing glass was not seen.

Further, when the relationship between the crystallizing temperature andthe crystalline phases is observed, as is clear particularly from FIG.6, the crystallizing temperature at which the eucryptite phase isproduced is shifted to a far higher temperature side in the use ofComposition C as compared with a case that no ZrO2 is added. As aresult, the temperature at which the spodumene phase is produced furtherrises. It is seen that the production amount of the eucryptite phaseconspicuously decreases when the crystallizing temperature lowers fromabout 790° C. to about 770° C., and simultaneously the lithium silicatephase increases and the Al₂ O₃ phase comes to be produced. From theseresults, it is seen that the center line average surface roughness afterthe fine polishing conspicuously lowers with respect to the crystallizedglass in which the relative ratio of the eucryptite phase is not morethan 50.

In Composition D, SiO₂ is slightly increased and Li₂ O is slightlydecreased as compared with Composition C, but similar results areobtained. In Composition E, 0.1 wt % of Fe₂ O₃ was further added and K₂O was decreased by 0.1 wt % as compared with Composition D, but similarresults are obtained.

Producing Examples Using Composition F and Evaluations

As mentioned above, an experimental sample made of a crystallized glasswas produced by using a raw material of Composition F, and the centerline average surface roughness and crystalline phases were measured. Inthis composition, SiO₂ was reduced to 71.0 wt % and 3.5 wt % of MgO wasadded instead. When the crystallization was effected at 750° C, neithereucryptite phase nor spodumene phase were observed from a X-raydiffraction pattern, but an α-quartz phase was observed therein. Thecenter line average surface roughness after the polishing was 18 Å.

The observation of the fine polished surface through an electronmicroscope revealed secondary particles of α-quartz at the polishedsurface. Since the aggregated secondary particles have higher hardnessthan the other crystalline phases and the grain boundary, it wasdifficult to obtain a higher flatness.

Observation with the Electron Microscope

Each experimental sample mentioned above was polished, and then etchedin a 5 wt % solution of hydrofluoric acid for one minute. The etchedsurface was observed with the electron microscope. With respect toexperimental samples, an experimental sample using Composition C andtreated at a crystallizing temperature of 750° C. is shown in FIG. 7,and one using Composition C and treated at a crystallizing temperatureof 790° C. is shown in FIG. 8, whereas FIG. 9 shows an experimentalsample using Composition A and treated at 750° C., and FIG. 10 shows oneusing Composition F and treated at a crystallizing temperature of 750°C.

As is seen from FIG. 8, when the crystallizing temperature was 790° C.,the secondary particles or the aggregated particles were produced due tothe growth of the eucryptite phase, and it became difficult to improvethe center line average surface roughness owing to difference inhardness between the secondary particles and the lithium disilicatephase. To the contrary, as is seen from FIG. 7, as the eucryptite phasedecreases, the above secondary particles disappear. As is seen from FIG.6, a very small amount of the eucryptite phase still remains even inthis state, but since such an eucryptite phase is not grown, it isconsidered that the phase does not influence the surface roughness. Theabove phenomenon could be clearly read from other scanning type electronmicroscope photographs not shown.

In FIGS. 9 and 10, the production of the secondary particles made of theeucryptite phase or the α-quartz phase is also seen.

Measurement Results of Strength

An experimental sample having a dimension of 4×40×3 mm was cut out froma crystallized glass using a raw material of Composition C and treatedat a crystallizing temperature of 730° C., and its four point bendingstrength was measured at room temperature according to JIS R1601. As aresult, the bending strength was 240 MPa. Further, an experimentalsample having a dimension of 4×40×3 mm was cut out from a crystallizedglass using a raw material of Composition A and treated at acrystallizing temperature of 730° C., and its four point bendingstrength was measured at room temperature according to JIS R1601. As aresult, the bending strength was 220 MPa.

Further, four point bending strength of a crystallized glass obtained byemploying a raw material of Composition A and a crystallizingtemperature of 700° C. was measured at room temperature according to thesame method as mentioned above. As a result, the bending strength was150 MPa.

Next, examples in which a texture was formed on a magnetic discsubstrate body according to the second aspect with laser beam will bedescribed.

EXAMPLE 6

Powdery compounds containing respective metals were mixed to give acompounding ratio of 76.1 wt % of SiO₂, 9.9 wt % of Li₂ O, 7.1 wt % ofAl₂ O₃, 2.8 wt % of K₂ O, 2.0 wt % of ZrO₂, 1.9 wt % of P₂ O₅, and 0.2wt % of Sb₂ O₃. At that time, an additive shown in Table 10 was added ata proportion (parts by weight) shown in FIG. 10 relative to 100 parts byweight of the resulting mixed powder when calculated in the form of anoxide. The resulting mixture was placed in a platinum crucible, andmelted by heating at 1450° C. for 5 hours. The melt was press molded,thereby obtaining a glass preform having a round disc shape.

Crystal nuclei were formed by holding the resulting glass preform at520° C. in an nitrogen atmosphere for one hour, and then the glass washeated at a heating rate of 100° C./hour, held at 730° C. for 2 hours,and cooled to room temperature. Thereby, a substrate material made of acrystallized glass was obtained.

A crystalline phase of this substrate material was identified by theX-ray diffraction method using a Kα line of copper. As a result, onlycrystalline phase of lithium disilicate was observed with respect to theexperimental samples excluding Experimental No. 6-18.

In Experimental No. 6-18, the lithium disilicate phase and theβ-eucryptite phase were observed. At that time, the intensity of thepeak (2θ=26.1°) of the β-eucryptite phase was 200, when the intensity ofthe peak (2θ=24.8°) of the lithium disilicate phase was taken as 100.Then, when the crystallization was effected at a lowered crystallizingtemperature of 680° C., the intensity of the peak of the β-eucryptitephase was 35, when the intensity of the peak of the lithium disilicatephase was taken as 100. Therefore, this sample was used.

Opposite surfaces of each substrate material were plane-polished to aflatness of 8 μm in a thickness of 0.75 mm by using a diamond grindingstone. Then, the polished opposite surfaces were lapped by using GCgrinding particles, thereby obtaining a finely polished body in athickness of 0.64 mm. Thereafter, the finely polished body was finishpolished to a thickness of 0.635 mm by using cerium oxide grindingparticles, thereby obtaining a magnetic disc substrate. The center lineaverage surface roughness of the thus treated surfaces was 6 to 9 Å, andtheir flatness was within 5 μm.

The surface of the above magnetic disc substrate was worked byirradiating a YAG fourth harmonic pulse laser beam (wavelength: 266 nm,pulse oscillation, Q switch) thereon. At that time, the workingcondition was that the oscillation frequency was 2 kHz. the pulse width25 nanoseconds, spot diameter 20 μm, and working speed 200 mm/sec withrespect to the laser. While the output of the laser was being measuredby a calorie meter type power meter, that output was varied and workingwas effected. The shape of the worked portion at which the laser beamwas irradiated was measured by a light-interfering type surfaceroughness meter and a 3-dimension contacting needle type shape measuringunit.

The worked shapes obtained are shown in Table 10. With respect to theworked trace having a protuberant shape, its height is shown.

                                      TABLE 10                                    __________________________________________________________________________               Addition amount                                                                       Laser      Height of                                         Experi-             (part by  output  protuberances                           mental No. Additive    weight) (mw)  Worked shape (Å)                   __________________________________________________________________________     6-1  none 0.00    200 no worked                                                    trace                                                                      6-2      Fe.sub.2 O.sub.3        0.05        40      hole                     6-3      Fe.sub.2 O.sub.3        0.05        36 protuberance       840        6-4      Fe.sub.2 O.sub.3        0.05        32 protuberance       430        6-5      Fe.sub.2 O.sub.3        0.05        30 protuberance       200        6-6      Fe.sub.2 O.sub.3        0.05        28 protuberance       150        6-7 Fe.sub.2 O.sub.3 0.05 24 no worked                                                                   trace                                              6-8      Fe.sub.2 O.sub.3        0.20        28      hole                     6-9      Fe.sub.2 O.sub.3        0.20        24 protuberance      1500       6-10      Fe.sub.2 O.sub.3        0.20        20 protuberance       900       6-11      Fe.sub.2 O.sub.3        0.20        18 protuberance       270       6-12      Fe.sub.2 O.sub.3        0.20        16 protuberance       120       6-13      Fe.sub.2 O.sub.3        0.20        14   no worked                      trace                                                                     6-14      Fe.sub.2 O.sub.3        0.15        18 protuberance       200       6-15      Fe.sub.2 O.sub.3        0.32         8 protuberance       180       6-16      Fe.sub.2 O.sub.3        0.48         2 protuberance       150       6-17      Fe.sub.2 O.sub.3        1.60       0.5 protuberance       200       6-18      Fe.sub.2 O.sub.3        3.20       0.5      hole                  __________________________________________________________________________

With respect to Experimental No. 6-1 in which no additive was added,working could not be effected even by raising the output of the laser toits maximum.

With respect to the crystallized glasses in which an additive was addedat a respective proportion, excluding Experimental No. 6-1, the shape ofthe working trace was "no worked trace" when the output of the laser wassmall, and took a swelled protuberant shape when the output of the laserwas large. Further, when the output of the laser was raised a hole wasformed. At that time, although a ring-shaped swelled portion was formedaround the hole, the height of the swelled portion was around 500 Åirrespective of the output of the laser. Thus, such crystallized glassescould not utilized for the texture having a target height of not be morethan 200 Å.

When the swelled protuberant shape was obtained, the height of theswelled portion was proportional to the output of the laser. Theprotuberances having the target height of not more than 200 Å could beobtained with the addition of an additive in a range of 0.05 parts byweight to 1.60 parts by weight.

As the addition amount of Fe₂ O₃ was increased, the output of the laserrequired for obtaining the swelled protuberances decreased and its rangewas narrowed. In Experimental No. 6-18 in which 3.20 parts by weight ofFe₂ O₃ was added, a hole was formed in a worked portion at an output of0.5 mW. It is not practical to reduce the output of laser to less thanthis level, and the addition amount is preferably not more than 3 partsby weight.

EXAMPLE 7

Glass substrates for magnetic discs were produced by the same method asin Example 6, provided that the kind and the amount of the additive werevaried as shown in Table 11.

The crystalline phase of the resulting substrate material was identifiedby the X-ray diffraction method using the Kα line of copper, and onlythe lithium disilicate was observed in each experimental samples.

The substrate material was finished to a magnetic disc substrate byworking in the same manner as in Example 6. The center line averagesurface roughness Ra of the surface thereof was 7 to 8 Å, and theflatness was within 4 μm.

Next, the substrate was worked by irradiating a YAG fourth harmonicpulse laser beam thereupon, and the shape of the resulting workedportion was evaluated.

                                      TABLE 11                                    __________________________________________________________________________               Addition amount                                                                       Laser      Height of                                         Experi-       (part by output  protuberances                                  mental No. Additive weight) (mw) Worked shape (Å)                       __________________________________________________________________________    7-1   V.sub.2 O.sub.5                                                                    0.50    18  protuberance                                                                         180                                               7-2      TiO.sub.2        0.48         60  protuberance      200                                           7-3      CeO.sub.2        1.00         60                                    protuberance      110                             7-4       CuO        0.50         40  protuberance      160                 __________________________________________________________________________

As shown above, the swelled protuberances having the height of not morethan 200 Å could be obtained, with respect to the crystallized glassesin which V₂ O₅, TiO₂, CeO₂ or CuO was added, by adjusting the output ofthe laser.

EXAMPLE 8

Powdery compounds containing respective metals were mixed to give acompounding ratio of 76.2 wt % of SiO₂, 10.0 wt % of Li₂ O, 6.5 wt % ofAl₂ O₃, 3.2 wt % of K₂ O , 22.5 wt % of ZrO₂, 1.5 wt % of P₂ O₅, and 0.1wt % of Sb₂ O₃. At that time, an additive shown in Table 12 was added ata proportion (parts by weight) shown in Table 12 relative to 100 partsby weight of the resulting mixed powder when calculated in the form ofan oxide.

With respect to Cr₂ O₃, it was added in the form of K₂ Cr₂ O₇ to giventhe parts by weight of Cr₂ O₃ shown in Table 12.

The resulting mixture was placed in a platinum crucible, and melted byheating at 1450° C. for 5 hours. The melt was press molded, therebyobtaining a glass preform having a round disc shape.

                  TABLE 12                                                        ______________________________________                                               Additive and added rate                                                Experimental                                                                           (part by weight)                                                     No.      Cr.sub.2 O.sub.3                                                                       CoO    MnO.sub.2                                                                            NiO  CuO   total                              ______________________________________                                         8-1     0.78     0.10   1.20              2.08                                  8-2     0.97    0.13  1.50                  2.60                              8-3     1.16    0.15  1.80                  3.11                              8-4     1.36    0.18  2.10                  3.64                              8-5     1.55    0.20  2.40                  4.15                              8-6     0.49    0.13  1.50                  2.12                              8-7     0.29    0.13  1.50                  1.92                              8-8               1.00                          1.00                          8-9               2.00                          2.00                         8-10               2.50                          2.50                         8-11                               2.00          2.00                         8-12                                       3.00  3.00                       ______________________________________                                    

Next, an inner diameter and an outer diameter of the glass plate wereworked in the same procedure as in Example 6, and the glass preform wascrystallized. At that time, the crystallizing temperature was variedbetween 680° C. and 760° C.

The crystalline phase was identified in the same manner as in Example 6by the X-ray diffraction method.

In Experimental samples, as the crystallizing temperature increased,there was tendency that the intensity of the peak of the β-eucryptitephase. When the crystallization was effected at the same temperature,there was tendency in Experimental Nos. 8-1 to 8-5 and 8-8 to 8-10 thatas the addition amount was increased, the intensity of the peak of theβ-eucryptite phase increased. In experiments were used samples in whichthe intensity of the peak (2θ=26.1°) of the β-eucryptite phase was notmore than 40, when the intensity of the peak (2θ=24.8°) of the lithiumdisilicate phase was taken as 100.

The sample was finish worked to a magnetic disc substrate in the samemanner as in Example 6. The center line average surface roughness Ra andthe flatness of the finished surface were 7-10 Å and within 5 μmrespectively.

The surface of the above magnetic disc substrate was worked byirradiating a YAG secondary harmonic pulse laser beam (wavelength: 532nm, CW oscillation, Q switch) thereon. At that time, the workingcondition was that the oscillation frequency was 1 kHz, the pulse width75 nano-seconds, spot diameter 20 μm, and working speed 40 mm/sec withrespect to the laser. While the output of the laser was being measuredby a calorie meter type power meter, that output was varied and workingwas effected.

The shape of the worked portion at which the laser beam was irradiatedwas measured in the same manner as in Example 6, and measurement resultsare shown in Table 13.

                  TABLE 13                                                        ______________________________________                                        Experiment  Laser output                                                                            Height of protuberances                                   No.         (mw)                (Å)                                     ______________________________________                                         8-1        120       hole                                                       8-1         100                2090                                           8-1          85                 910                                           8-1          70                 150                                           8-1          60          no worked trace                                      8-2          70                hone                                           8-2          60                2550                                           8-2          55                 470                                           8-2          45                 190                                           8-2          30          no worked trace                                      8-3          55                hole                                           8-3          45                3220                                           8-3          30                 620                                           8-3          23                 130                                           8-3          15          no worked trace                                      8-4           5                 180                                           8-5           1                 200                                           8-6          50                 180                                           8-7          50                 160                                           8-8         120                 130                                           8-9          50                 150                                          8-10         50                 200                                           8-11        150                 120                                           8-12        200                 100                                         ______________________________________                                    

As the output of the laser increased the shape of the worked portionobtained by working the crystallized glass with use of the YAG secondaryharmonic pulse laser beam successively changed from "no worked trace","swelled protuberant shape" and "hole bored" in this order as in the useof the YAG fourth harmonic pulse laser beam.

Even when the YAG secondary harmonic pulse laser beam was used, thetexture having swelled protuberances of a target height of not more than200 Å could be obtained by adjusting the output of the laser dependingupon the kind and the amount of the additive.

EXAMPLE 9

A sample of Experimental No. 8-1 in Example 8 was worked by irradiatingan argon laser beam (wave-length: 514 nm, CW oscillation) thereon. Atthat time, the working condition was that the output was 700 mW, spotdiameter 50 μm, and working speed 4 mm/sec with respect to the laser.The working was effected in the state that the focus of the laser beamwas moved upwardly of the sample.

The worked portion at which the laser beam was irradiated was swelled inthe form of a protuberance in every Experimental sample where thefocus-moved amount ranged from 0 mm to 1.2 mm (No hole was bored).

The swelled shape was measured by the surface roughness meter, andresults in Table 14 were obtained.

                  TABLE 14                                                        ______________________________________                                        Focus-deviated amount                                                                        Height of protuberances                                          (mm)                    (Å)                                             ______________________________________                                        0              33750                                                            0.50                  16680                                                   1.00                    4860                                                  1.10                    1670                                                  1.13                    830                                                   1.15                    330                                                   1.20                    160                                                   1.22             no worked trace                                            ______________________________________                                    

As seen above, even when the argon laser CW beam was used, the texturehaving swelled protuberances of a target height of not more than 200 Åcould be obtained by adjusting the density of the energy of the laserbeam at the surface of the sample.

EXAMPLE 10

Powdery compounds containing respective metals were mixed to give acompounding ratio of 76.1 wt % of SiO₂, 9.9 wt % of Li₂ O, 6.1 wt % ofAl₂ O₃, 2.8 wt % of K₂ O, 3.0 wt % of ZrO₂, 1.9 wt % of P₂ O₅, and 0.2wt % of Sb₂ O₃. At that time, an additive shown in Table 15 was added ata proportion (parts by weight) shown in Table 15 relative to 100 partsby weight of the resulting mixed powder. The resulting mixture wasplaced in a platinum crucible, and melted by heating at 1450° C. for 5hours. The melt was press molded, thereby obtaining a glass preformhaving a round disc shape. This round discoidal glass preform was groundto a dimension of 15 mm×15 mm×0.8 mm.

Crystal nuclei were formed by holding the resulting glass preform at520° C. in an nitrogen atmosphere for one hour, and then the glass washeated at a heating rate of 100° C./hour, held at 710° C. to 730° C. for4 hours, and cooled to room temperature. Thereby, a crystallized glasswas obtained.

A crystalline phase of this substrate material was identified by theX-ray diffraction method using a Kα line of copper. The intensity of thepeak (2θ=26.1°) of the β-eucryptite phase is shown in Table 15, when theintensity of the peak (2θ=24.8°) of the lithium disilicate phase istaken as 100.

The substrate material was finish polished to a thickness of 0.635 mm byusing cerium oxide grinding particles as in the same manner as inExample 6. The surface roughness of the thus treated surfaces wasmeasured by using the surface roughness meter with a contacting needleof 0.5 μm. The thus obtained center line average surface roughnesses Raare shown in Table 15.

                                      TABLE 15                                    __________________________________________________________________________                                    Surface                                         Additive and added rate     Crystallizing  Ratio in roughness                 (part by weight)         temperature     peak       Ra                      Fe.sub.2 O.sub.3                                                                      Cr.sub.2 O.sub.3                                                                  CoO MnO.sub.2                                                                          (° C.)                                                                       intensity                                                                          (Å)                                       __________________________________________________________________________     10-1                                                                             0.08             730   0    7                                                10-2   1.60                                 730          17         6                                        10-3   2.40                                                                     730          31        10                    *10-4   3.20                                 730         200        27        10-5   3.20   5.00                        730          22         8                                          10-6                    1.50                                                    730          18         9                    10-7                    2.50                730          20        10                                        *10-8                    3.50                                                    730          76        16                   *10-9            0.40  3.50                730          55        13                                         10-10                            1.00                                           710          11         7                    10-11                            2.00       710          28        10                                        10-12                            3.00                                           710         46        10                    *10-13                            4.00       710          87        17                                        10-14           3.00           4.00                                           710          32         9                      10-15           2.88  1.13   1.50       730          10         8                                            10-16           1.94  1.13   1.50                                           730          28         9                        10-17                   1.13   1.50       730          41        10                                          10-18           2.16  0.15   1.80                                           710          0          7                        10-19           1.56  0.15   1.80       710          26         9                                            10-20           1.28  0.15   1.80                                           710          29         9                     __________________________________________________________________________

As is seen from Experimental Nos. 10-1, 10-2, 10-3, 10-4, 10-6, 10-7,10-8, 10-10, 10-11, 10-12 and 10-13, there is tendency that theintensity of the peak of the β-eucryptite phase increases in proportionto the addition amount of the oxide added.

In order to suppress the center line average surface roughness to notmore than 10 Å, the total or a single amount of the oxide(s) excludingchromium oxide is preferably not more than 3 parts by weight.

However, as is seen from Experimental Nos.10-5, 10-9, 10-14, 10-15,10-16, 10-18, 10-19 and 10-20, the addition of chromium oxide functionsto suppress the production of the β-eucryptite phase. The additionamount of chromium oxide is preferably not more than 10 parts by weight.If the addition amount exceeds 10 parts by weight, the amount of thecrystals on crystallizing decreases to lower strength.

Even if the oxide(s) excluding chromium oxide was added in a totalamount of not less than 3 parts by weight, the effect of suppressing theproduction of the β-eucryptite phase with chromium oxide can be utilizedwhen chromium oxide is added in an amount of 0.5 parts by weight. Theabove oxides may be added in a total amount of not more than 10 parts byweight.

What is claimed is:
 1. A magnetic disc substrate comprising a magneticdisc substrate body comprising glass, at least one metal elementcontained, in the form of ions, in at least a surface portion of themagnetic disc substrate body, said metal clement absorbing light havinga wavelength of 1600 nm or less, and a textured area formed on a surfaceof the magnetic disc substrate body, wherein the surface roughness ofsaid textured area is greater than that of an untextured area of saidsurface.
 2. The magnetic disc substrate set forth in claim 1, whereinthe ions of said metal are one or more kinds of ions selected from thegroup consisting of silver ions, copper (I) ions, thallium ions, ironions, chromium ions and cobalt ions.
 3. The magnetic disc substrate setforth in claim 1, wherein said magnetic disc substrate body is comprisesa Li₂ O--Al₂ O₃ --SiO₂ based crystallized glass which contains 65 to 85wt % of SiO₂, 8 to 15 wt % of Li₂ O, 2 to 8 wt % of Al₂ O₃, 1 to 5 wt %of P₂ O₅ and 1 to 10 wt % of ZrO₂ and has lithium disilicate (Li₂O.2SiO₂) as a main crystalline phase.
 4. The magnetic disc substrate setforth in claim 3, wherein one or more kinds of metallic elements arecontained in the crystallized glass in the form of oxides, the oxidesare other than chromium oxides, and an addition amount of the oxides isnot more than 0.01 parts by weight but not more than 3 parts by weight.5. The magnetic disc substrate set forth in claim 3, wherein the metalelement is contained in the crystallized glass in the form of an oxide,said oxide includes at least chromium oxide, and an addition amount ofchromium oxide is not less than 0.01 parts by weight but not more than10 parts by weight.
 6. The magnetic disc substrate according to claim 5,wherein an addition amount of chromium oxide is not less than 0.5 partsby weight but not more than 10 parts by weight, and the total additionamount of the oxides is not less than 3 parts by weight but not morethan 10 parts by weight.
 7. The magnetic disc substrate set forth inclaim 1, wherein said untextured area includes a reading/writing zonefor recording and reproducing signals, and said textured area forms alanding zone that a slider of a magnetic head is to contact when amagnetic disc drive is stopped, and wherein the texture is constitutedby bumps having heights in a range of 50 to 200 Å formed in the landingzone, and an area of top portions of the bumps is 2 to 5% of the totalarea of the landing zone.
 8. A magnetic disc comprising a magnetic discsubstrate set forth in claim 1, further comprising a magnetic filmformed on the magnetic disc substrate.
 9. The magnetic disc substrateset forth in claim 1, wherein said textured area serves as a landingarea for a magnetic head and the center line average surface roughness(Ra) of said untextured area is not more than 10 Å.
 10. A Li₂ O--Al₂ O₃--SiO₂ based crystallized glass comprising 65 to 85 wt % of SiO₂, 8 to15 wt % of Li₂ O, 2 to 8 wt % of Al₂ O₃, 1 to 5 wt % of P₂ O₅ and 1 to10 wt % of ZrO₂ and having lithium disilicate (Li₂ O.2SiO₂) as a maincrystalline phase, wherein the glass enables formation of substrateshaving a flat and smooth surface with a center line average surfaceroughness (Ra) after finely polishing of not more than 10 Å.
 11. Thecrystallized glass set forth in claim 10, wherein substantially no MgOis contained in a composition of the crystallized glass.
 12. Thecrystallized glass set forth in claim 10, wherein with respect tocrystalline phases constituting the crystallized glass, a sum of anintensity of an X-ray diffraction peak of an eucryptite (Li₂ O.Al₂O₃.2SiO₂) phase and an intensity of an X-ray diffraction peak of aspodumene phase (Li₂ O.Al₂ O₃.4SiO₂) phase is not more than 50, when anintensity of an X-ray diffraction peak of a lithium disilicate (Li₂O.2SiO₂) phase is taken as
 100. 13. The crystallized glass set forth inclaim 12, wherein substantially no MgO is contained in a composition ofthe crystallized glass.
 14. A process for producing a Li₂ O--Al₂ O₃--SiO₂ based crystallized glass, said process comprising the step ofcrystallizing a glass preform which contains 65 to 85 wt % of SiO₂, 8 to15 wt % of Li₂ O, 2 to 8 wt % of Al₂ O₃, 1 to 5 wt % of P₂ O₅ and 1 to10 wt % of ZrO₂ and has lithium disilicate (Li₂ O.2SiO₂) as a maincrystalline phase, by heating it such that a maximum temperature in saidheating is 680° C. to 770° C., wherein the glass enables formation ofsubstrates having a flat and smooth surface with a center line averagesurface roughness (Ra) after finely polishing of not more than 10 Å. 15.A magnetic disc substrate comprising a Li₂ O--Al₂ O₃ --SiO₂ basedcrystallized glass comprising 65 to 85 wt % of SiO₂, 8 to 15 wt % of Li₂O, 2 to 8 wt % of Al₂ O₃, 1 to 5 wt % of P₂ O₅ and 1 to 10 wt % of ZrO₂and having lithium disilicate (Li₂ O.2SiO₂) as a main crystalline phase,wherein said substrate has a flat and smooth surface with a center lineaverage surface roughness (Ra) of not more than 10 Å.
 16. The magneticdisc substrate of claim 15, wherein with respect to crystalline phasesconstituting the crystallized glass, a sum of an intensity of an X-raydiffraction peak of a eucryptite (Li₂ O.Al₂ O₃.2SiO₂) phase and anintensity of an X-ray diffraction peak of a spodumene (Li₂ O.Al₂O₃.4SiO₂) phase is not more than 50, when an intensity of an X-raydiffraction peak of a lithium disilicate (Li₂ O.2SiO₂) phase is taken as100.
 17. The crystallized glass set forth in claim 16, whereinsubstantially no MgO is contained in a composition of the crystallizedglass.
 18. The magnetic disc substrate as set forth in claim 15, whereinsubstantially no MgO is contained in a composition of the crystallizedglass.
 19. A magnetic disc, comprising:a Li₂ O--Al₂ O₃ --SiO₂ basedcrystallized glass comprising 65 to 85 wt % of SiO₂, 8 to 15 wt % of Li₂O, 2 to 8 wt % of Al₂ O₃, 1 to 5 wt % of P₂ O₅ and 1 to 10 wt % of ZrO₂and having lithium disilicate (Li₂ O.2SiO₂) as a main crystalline phase,wherein said substrate has a flat and smooth surface with a center lineaverage surface roughness (Ra) of not more than 10 Å; an underlying filmformed on the flat and smooth surface of the magnetic disc substrate;and a metallic magnetic layer formed on the underlying film.
 20. Amagnetic disc substrate comprising a magnetic disc substrate bodycomprising:a Li₂ O--Al₂ O₃ --SiO₂ based crystallized glass comprising 65to 85 wt % of SiO₂, 8 to 15 wt % of Li₂ O, 2 to 8 wt % of Al₂ O₃, 1 to 5wt % of P₂ O₅ and 1 to 10 wt % of ZrO₂ and having lithium disilicate(Li₂ O--2SiO₂) as a main crystalline phase; at least one metal elementcontained in at least a surface portion of the magnetic disc substratebody, said metal element absorbing light having a wavelength of 1600 nmor less; and a textured area formed on a surface of the magnetic discsubstrate body, wherein the surface roughness of said textured area isgreater than that of an untextured area of said surface.
 21. Themagnetic disc substrate set forth in claim 20, wherein one or more kidof metallic elements are contained in the crystallized glass in the formof oxides, the oxides are other than chromium oxides, and an additionamount of the oxides is more than 0.01 parts by weight but not more than3 parts by weight.
 22. The magnetic disc substrate set forth in claim20, wherein the metal element is contained in the crystallized glass inthe form of an oxide, said oxide includes at least chromium oxide, andan addition amount of chromium oxide is not less than 0.01 parts byweight but not more than 10 parts by weight.
 23. The magnetic discsubstrate according to claim 22, wherein an addition amount of chromiumoxide is not less than 0.5 parts by weight but not more than 10 parts byweight, and the total addition amount of the oxides is not less than 3parts by weight but not more than 10 parts by weight.